Conventionally, as a lithography technique for the fabrication of fine semiconductors, such as a semiconductor memory or a logic circuit, projection reduction exposure using ultraviolet light has been employed.
The minimum size which can be transferred by the projection reduction exposure is proportional to the wavelength of light of the transfer, and inversely proportional to the numerical aperture of the projection optical system. Accordingly, to transfer a fine circuit pattern, light having a short wavelength such as a mercury lamp i-ray (wavelength: 365 nm), a KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm) are employed. Thus, the wavelength of ultraviolet light has been shortened.
However, as finer semiconductor devices are rapidly developed, the transfer of such finer devices cannot be handled without difficulty in the lithography using ultraviolet light. Accordingly, to efficiently print a very fine circuit pattern, less than 1 μm, a projection reduction exposure apparatus using extreme ultraviolet light (EUV light) having a wavelength of 10 to 15 nm, which is further shorter than that of the ultraviolet ray, has been developed.
In an EUV light area, as the amount of absorption by material is very large, a lens optical system utilizing light refraction, which is used for visible and ultraviolet light, is impractical. Accordingly, the exposure apparatus using EUV light employs a reflection optical system. In this case, a reflective type reticle where a pattern to be transferred is formed by light absorbing material on a mirror is employed as a plate.
As a reflection optical device constructing the EUV exposure apparatus, a multilayer mirror and an oblique incidence total reflection mirror are known. In the EUV area, as a substantial part of the refractive index is slightly less than 1, total reflection occurs by using EUV light as oblique incidence as close to the surface as possible. Generally, in oblique incidence, within several degrees from the surface, a high refractive index of several tens of % or higher can be obtained. However, as the freedom of optical design is limited, it is difficult to use the total reflection mirror in the projection optical system.
The mirror for the EUV light used at an incident angle close to direct incidence is a multilayer mirror where two types of materials having different optical constants are alternately laminated. For example, molybdenum and silicon are alternately laminated on the surface of a glass substrate, which is ground to have a precise surface shape. The thickness of the molybdenum layer is, e.g., 0.2 nm, that of the silicon layer, e.g., 0.5 nm, and the number of layers is about twenty pairs. The sum of the thicknesses of the two types of layers is called a film period. In the above example, as the film period, 0.2 nm+0.5 nm=0.7 nm holds.
When the EUV light is incident on the multilayer mirror, EUV light having a particular wavelength is reflected. Assuming that the incident angle is θ, the EUV light wavelength, λ, and the film period, d, only narrow band EUV light, mainly having the wavelength λ approximately satisfying the relation by Bragg's equation2×d×sin θ=λis efficiently reflected. The bandwidth at this time is about 0.6 to 1 nm.
The reflectivity of the reflected EUV light is about 0.7 at the maximum. EUV light, which has not been reflected, is absorbed in the multilayer or the substrate, and most of the energy of the light becomes heat.
As light loss of the multilayer mirror is greater in comparison with a visible light mirror, the number of mirrors must be a minimum number. To realize a wide exposure area with a small number of mirrors, employed is a method for transfer (scan exposure) in a wide area by simultaneously scanning a reticle and a wafer using only a slim ring area (ring field) away from an optical axis by a predetermined distance.
FIG. 6 is a schematic diagram of the conventional projection reduction exposure apparatus using EUV light. The exposure apparatus comprises an EUV light source 50, an illumination optical system 60, a reflective type reticle 81, a projection optical system 70, a reticle stage 80, a wafer stage 85, an off-axis alignment optical system (detection mechanism) 90, a vacuum system, and the like.
The EUV light source 50 is, e.g., a laser plasma light source. Light from a high-intensity pulse laser 53 is gathered by a light gathering lens 54, emitted on a target material placed in a vacuum container 52 supplied from a target supply device 51, to cause high temperature plasma 55, and EUV light having a wavelength of, e.g., about 13 nm, radiated from the plasma is utilized. As the target material, a metal thin film, inertia gas, a liquid drop, or the like, is used. The target material is supplied by gas jet means, or the like, into the vacuum container 52. To increase the mean intensity of the radiated EUV light, it is preferable that the repetition frequency of the pulse laser 53 is high. Generally, the pulse laser is operated by a several kHz repetition frequency.
The illumination optical system 60 comprises plural multilayer or oblique incidence first to third mirrors 61 to 63, an optical integrator 64, and the like. The first-stage light gathering mirror 61 corrects EUV light approximately isotropically radiated from the laser plasma 53. The optical integrator 64 uniformly illuminates a mask with a predetermined numerical aperture. Further, an aperture 65 to limit an illuminated area of the reticle surface to a circular shape is provided in the position of the illumination optical system 60 conjugate with the reticle 81.
The projection optical system 70 uses plural mirrors 71 to 74. As the number of mirrors is small, the efficiency of use of EUV light is high, however, the aberration cannot be easily corrected. The number of mirrors necessary for aberration correction is about four to six. The shape of the mirror reflection surface is a spherical surface such as a convex or concave surface or an aspherical surface. The numerical aperture NA is about 0.1 to 0.3.
The mirror is obtained by grinding and polishing a substrate of a material having a high rigidity and hardness and a low thermal expansion rate, such as low-expansion glass or silicon carbine, to form a predetermined reflection surface shape, then forming a multilayer film of molybdenum, silicon, and the like, on the reflection surface. If the incident angle is not constant depending on a position within the mirror surface, as it is apparent from the above-described Bragg's equation, the wavelength of EUV light having the reflectivity, which increases depending on the position of the multilayer film having a constant film period, is shifted. Accordingly, the mirror surface must have a film period distribution to attain efficient reflection of the EUV light of the same wavelength within the mirror surface.
The reticle stage 80 and the wafer stage 85 have a mechanism to scan in synchronization with each other at a speed rate proportional to a reduction scaling factor. In the reticle 81 or the wafer 86 surface, a scanning direction is X, a direction vertical to the scanning direction is Y, and a direction vertical to the reticle 81 or the wafer 86 surface is Z.
The reticle 81 is held on a reticle chuck 82 on the reticle stage 80. The reticle stage 80 has a mechanism to move in the direction X at a high speed. Further, the reticle stage has a mechanism to slightly move in the directions X, Y and Z and rotational directions about the respective axes for positioning of the reticle 81. The position and posture of the reticle stage 80 is measured by a laser interferometer, and the position and the posture are controlled in accordance with the result of the measurement.
The wafer 86 is held on the wafer stage 85 by the wafer chuck 88. The wafer stage 85 has a mechanism similar to that of the reticle stage 80 to move in the direction X at a high speed. Further, the wafer stage has a mechanism to slightly move in the directions X, Y and Z and rotational directions about the respective axes for positioning of the wafer. The position and posture of the wafer stage 85 is measured by a laser interferometer, and the position and the posture are controlled in accordance with the result of measurement.
The alignment detection mechanism 90, as in the case of, e.g., an ArF exposure apparatus, performs wafer alignment by an off-axis bright field illumination image processing detection system while holding a predetermined baseline amount.
Further, a focus position in the direction Z is measured by a focus position detection mechanism 91, and the position and angle of the wafer stage 85 are controlled, thereby the wafer surface is held in an image-formation position by the projection optical system 70 during exposure.
When one scan exposure has been completed on the wafer 86, the wafer stage 85 step-moves in the directions X and Y to the next scan exposure start position. Again, the reticle stage 80 and the wafer stage 85 scan in synchronization with each other in the direction X at the speed rate proportional to the reduction scaling factor of the projection optical system 70.
In this manner, the synchronized scanning in the status wherein a reduced projection image of the reticle 81 is formed on the wafer 86 is repeated (step and scan). Thus, the transfer pattern of the reticle 81 is transferred onto the entire surface of the wafer 86.
The off-axis bright field illumination image processing detection system is used as the alignment detection mechanism as in the case of the ArF exposure apparatus, however, to address a requirement for finer semiconductor devices, alignment in higher precision must be realized. Accordingly, the stability of the baseline is required equally or more in comparison with the ArF exposure apparatus.
However, in the EUV exposure apparatus, a measurement system to automatically measure the baseline, especially, means for measuring the reticle and wafer, or the like, has not been proposed.
The automatic measuring system has not been proposed since a relative positional alignment (hereinafter, referred to as “TTL (Through The Lens) alignment”) between the reticle and the wafer via the projection optical system has the following problems.
In a case wherein the TTL alignment is performed in the EUV exposure apparatus, illumination light to detect an alignment mark (e.g., as the wavelength used is not EUV, it is non-exposure light) is reflected from the reflective type reticle and passed through the multilayer mirror optical system. The light illuminates a wafer alignment mark on the wafer, and then, reflected light from the wafer alignment mark is again passed through the multilayer mirror optical system and the reflective type reticle. Then, the alignment mark is detected by the alignment detection optical system having an image formation optical system and an image sensing device.
In this manner, if the TTL alignment is performed by the non-exposure light via the reflective type reticle and the multilayer mirror, as the reflective type reticle and the multilayer mirror are optimized to attain a high reflectivity by the EUV light, a sufficient reflectivity cannot be attained by the non-exposure illumination light. Accordingly, there is a possibility that high precision alignment cannot be performed.
Further, in the case of an off-axis method, the stability of the baseline is required. To attain the stability of the baseline, it is necessary to use a material having a high mechanical rigidity and low thermal sensitivity and to attain ultra stable heat distribution, which increase the cost of the apparatus.